1. Field of the Invention
This invention relates to a method and device for the synchronous control of a static relay by an high frequency signal, that is a signal having a frequency which is high with respect to the frequency of the mains.
2. Description of the Prior Art
A synchronous switch is a static device adapted to open or to close a charge circuit fed by a rectified alternating or alternating power supply, in synchronism with a voltage passing a determined value close to a zero value and in relationship with a control signal. In other words, the invention aims at synchronizing the triggering of the switch in order that its operation (closure or opening of a circuit) takes place at times when the instant power is relatively very low. For this purpose, the switch control is determined in order to satisfy the following conditions:
Closing close to the time of the first zero passing of the feed voltage which follows the closing order, PA1 Holding the closed state until the current becomes nul even if the closing order has been suppressed or if the voltage at the terminals of the switch increases above the threshold value, PA1 Opening of the circuit close to the time of the zero passing of the current and automatic re-closing immediately after this time as long as the closing order is maintained; or holding of the open state of the circuit if the closing order is suppressed until the time of the first zero passing of the feed voltage which follows the possible re-application of a closing order. PA1 its construction is very simple and cheap; PA1 the control signal source is practically disconnected from the charge circuit at the times when there is no triggering, which causes a lowering of the necessary control power; PA1 the variable impedance, which is preferencially comprised of a diode reverse biased by the rectified mains voltage, can be adapted to the input characteristics of the thyristor in a way which is substantially independent from the temperature and the environment; PA1 the control circuits do not cause any substantial parasitic current at low frequency in the main circuit which allows a very high resistor to be maintained at the terminals of the main switch when it is open.
Conventionally, in case the main current to be switched is a rectified alternating current, unidirectional switches of the "thyristor" type are used, and in case the feed voltage to be switched is an alternating voltage, either an unidirectional switch of the thyristor kind associated with a full-wave rectifier, or two unidirectional switches of the "thyristor" type connected in an antiparallel manner, or a bidirectional switch of the triac type is used.